Electromagnetic wave reflector and method for making same

ABSTRACT

In this antenna in an element in an electrically conductive material, the element comprises a gauze coated at least in part by a coating of thermoplastic material.

The present invention concerns reflectors of electromagnetic waves inelement in an electrically conductive material, preferably having aconvex face which defines a focal point for radio waves. These are moreoften antennae in the form of a spherical or parabolic dish, but theymay also have any other shape which provides focussing.

A metallic antenna produced by stamping in one or more passes, aspherical dish enabling signals from a satellite to be focussed to apoint, is already known. The metal surface is radio opaque tofrequencies transmitted by the satellite and permits a theoreticalreflection rate of 100%. Since the shape is obtained by stamping, thesheet has to undergo several surface treatments in order to preventcorrosion. These treatments, in particular the application of a primerand a paint, have to be repeated after stamping.

Antennae made of thermosetting material which require tooling designedto obtain the desired shape, in particular parabolic, are also disclosedby DE 3911445 A. A metallic frame is manually shaped in the tool. A lumpof material is then introduced into the tool above the frame. The toolis locked. A firing is carried out to polymerise the inserted material.After the firing, the tool is opened and the part is extracted from themould. Because of the moulding technology, deburring has to be carriedout, followed by sanding, application of a primer then a paint coat onthe two faces.

An antenna containing a metallic gauze coated in thermoplastic resin byinjection moulding is described in EP-595 418. The metallic gauze isgiven the desired shape before moulding is carried out. To shape ametallic gauze into a parabola with great accuracy is an operation thatis very difficult to achieve.

Painting technologies with powder coating do not permit a largevariation in colour without incurring disproportionate manufacturingcosts due to the need to clean the production tool when changing fromone colour to the other.

The invention overcomes these drawbacks and enables a reflector ofelectromagnetic waves to be obtained, which is protected againstcorrosion, much easier to manufacture and which in particular allowspossible use of a paint that is much easier and less expensive toimplement and thus eliminates any painting operation with powder coatingand the cleaning operations which this involves.

The object of the invention is a process for manufacturing a reflectorof electromagnetic waves having a shape providing focussing ofelectromagnetic waves, in which process an electrically conductivelattice is placed in a moulding tool, thermoplastic plastic material isplaced in the moulding tool and a reflector is demoulded, wherein

the lattice placed in the moulding tool has a shape that differs fromsaid shape providing focussing of electromagnetic waves, and

the thermoplastic material is injected into the moulding tool under sucha pressure that it forms the lattice into said shape providingfocussing.

The lattice no longer has to be shaped independently of the mouldingoperation. This shaping is carried out by the moulding operation itself

The lattice of electrically conductive material may comprise wiresarranged at a distance, preferably at a constant spacing, from eachother. For preference, the lattice is a gauze or a grid, in particularplanar.

Manufacture is effected by injection moulding a thermoplastic material,preferably onto a grid with a uniform mesh of an electrically conductivematerial. Injection is carried out at high pressure, for example inexcess of 50 bar and more often between 150 and 450 bar.

For preference, the process comprises

gripping between the two mould halves of a thermoplastic materialinjection mould a lattice of an electrically conductive material of sucha dimension that it overhangs the periphery of the die of the mould,

injecting thermoplastic material into the die,

demoulding an injected part from the periphery of that which overhangsthe lattice, and

cutting off the lattice level with the part.

The lattice is thus well supported during moulding and the curves of thelattice and of the coating are not off centre, everything being obtainedautomatically by the usual moulding operation. The coating of the wiresis often virtually complete and in any case occurs over more than 80% ofthe transverse section of the wires.

Ferrous and non-ferrous metals, among others, as well as carbon fibre ortheir oxides and particularly aluminium and stainless steel, can be usedas an electrically conductive material. The wire usually has a diameterof 0.005 to 0.5 mm, it being understood that the wires can also have atransverse section that is other than circular and that the valuesindicated above then apply to the largest dimension of the transversesection.

Polyethylene, polypropylene or other polyolefins, ABS, polycarbonate,polymethyl methacrylate, among others, as well as any other injectablethermoplastic material, can be used as thermoplastic material. Thethickness of the injected plastic material is generally between 0.5 mmand 1 cm.

Advantageously, the distance between the wires is between 0.003 mm and1.5 cm.

According to a particularly preferred embodiment, the desired colour forthe antenna is imparted to it by the fact the plastic material iscoloured in bulk. The antenna may also be transparent.

The object of the invention is also a reflector of electromagnetic wavescomprising a lattice of an electrically conductive material coated by acoating of thermoplastic material, having a central part and a border,characterised in that in the central part the lattice appears at pointson one of the faces of the coating, while it is buried in the coating onone part of the border.

Depending on the direction of injection, the lattice appears at pointson one or other of the faces of the border.

In the attached drawing, provided solely by way of an example:

FIG. 1 is a perspective view showing two mould halves according to theinvention, between which is inserted a metallic gauze;

FIG. 2 shows the metallic gauze at the stage where it is gripped betweenthe two mould halves;

FIG. 3 is an elevation view of an antenna according to the inventionbefore the gauze has been cut off;

FIG. 4 shows the antenna with an offcut of the gauze which has been cutoff;

FIG. 5 is a partial sectional view of an antenna according to theinvention, and

FIG. 6 is a partial sectional view of a variant of an antenna accordingto the invention.

A planar metallic gauze 1 is held on one of the two mould halves 2, 3 ofa thermoplastic material injection mould. FIG. 2 shows that when thegauze 1 is gripped between the two mould halves 2, 3, it overhangs theperiphery of the die 4 of the mould.

FIG. 3 illustrates the demoulded injected part 5 in which the gauze 1 isencapsulated, and which extends to a part 6. FIG. 4 shows that the part6 is cut off in the form of an offcut 6, leaving a parabolic antenna inthe shape of the die 4, which antenna, as shown by FIGS. 5 and 6,consists of the gauze 1 coated by a coating 7 of thermoplastic plasticsmaterial. The antenna contains a central part 8 of parabolic shape and aperipheral border 9. In the central part 8 the gauze appears at pointson one of the faces of the coating 7, while it is buried in the coating7 on at least one part of the border 9. The antenna of FIG. 5 has beenobtained when the direction of the injection current in the mould isdirected vertically from bottom to top, while in FIG. 6 it was directedvertically from top to bottom.

The following example illustrates the invention.

Polyethylene previously brought to a temperature of 260° C. so that itcan run through an injection point into the die 4 of the mould, whichhas the shape of the reflector to be produced, and into which a planaraluminium gauze is placed and which overhangs it. To fill the die apressure of 177 bar is applied to the plastics material, the closingforce of the two halves of the mould being 800 tonnes. 1.2 kg ofpolyethylene is injected into the die. The duration of injection is 7seconds. The injection pressure is 117 bar. On completion of injectionit is left to cool for 50 seconds, the mould is opened and the aluminiumwire gauze is cut off to obtain a reflector of electromagnetic wavesaccording to the invention.

What is claimed is:
 1. A process for manufacturing a reflector ofelectromagnetic waves having a shape providing focussing of theelectromagnetic waves, the process comprising: placing an electricallyconductive lattice in a thermoplastic material injection mould havingtwo mould halves, wherein the lattice placed in the moulding tool is aplane metallic gauze and has a shape that differs from said shapeproviding focussing of the electromagnetic waves; gripping between thetwo mould halves of the thermoplastic material injection mould theelectrically conductive lattice, wherein the lattice is of such adimension that a portion of the lattice overhangs the periphery of themould; injecting a thermoplastic material into the mould under such apressure that the thermoplastic material forms the lattice into saidshape providing focussing; demoulding an injected part from theperiphery of that which overhangs the lattice, and cutting off thelattice level with the part.
 2. Process according to claim 1, whereinsaid shape providing focussing is a parabolic shape.
 3. Processaccording to claim 1, wherein the injection pressure of thethermoplastic plastic material exceeds 50 bar.
 4. A reflector ofelectromagnetic waves comprising a lattice of an electrically conductivematerial coated by a coating of thermoplastic plastic material andhaving a central part and a border, wherein the central part the latticeappears on one of the faces of the coating, while it is buried in thecoating on one part of the border.
 5. A reflector according to claim 4,wherein the lattice appears on one of the faces of the border. 6.Reflector according to claim 4, wherein the thermoplastic plasticmaterial is coloured in bulk or is transparent.